The present invention relates to a low light level image pick-up tube arrangement. The invention applies more particularly to equipment comprising a pick-up tube or a television camera tube which is intended to operate with very limited ambient illumination in the visible range and possibly in the near infra-red range, the scene illumination possibly being equivalent to that produced by the residual night light.
Such arrangements are fitted in particular to industrial or military surveillance systems. As an example the arrangement may form part of a fire-control system so as to make possible night-time aiming and firing. In this type of application, which should not be looked upon as limiting, bright lights caused by firing appear in the field of view of the so-called "camera" device.
Owing on the one hand to the high ratio between the illumination produced by such extraneous sources and the ambient nighttime light level, and on the other hand to the extreme sensitivity of the camera tube incorporated in the device, difficulties arise the seriousness of which depends on the brightness of the sources in question, the length of time for which they persist and the area which they occupy in the observed field.
Unless special steps are taken, there is a danger of the target of a low-light-level camera tube being suddenly saturated over the whole or part thereof when the scene illumination exceeds permitted limits; the tube will then take a fairly long time, perhaps several seconds, before it will again produce a normal image. In connection with this drawback of saturation, it should also be mentioned that, where the light energy received is even more excessive, there is a risk of the target being destroyed, for which there is no remedy. Furthermore, the camera tube may incorporate a light image intensifier stage having a luminescent screen the input of which receives the incident radiation through an optical focussing lens. In the case of too strong an illumination, presistance on the screen may be too long and this may result in the image being lost until this so-called "afterglow" effect ceases with the return to the normal persistence level.
It is therefore helpful to provide the arrangement with means for protecting the tube against the effects of excessive illumination. It is also well to provide the arrangement with means for controlling the amplitude of the video signal so as to ensure that substantially optimum operating conditions are maintained when ambient illumination varies within a pre-established operating range.
According to known techniques, it is possible to regulate the video signal from the difference between the mean level of the video signal and a reference level which is equivalent to the tube operating under optimum conditions. This measurement gives an error signal which is used in a control loop to control the amplitude of a very high DC voltage supplying the tube and thus, by varying the gain of the tube the video signal is regulated. The regulation circuit presents a certain response time constant due to the measure of the mean video signal and is therefore only able to operate effectively when illumination is within a limited range.